1. Field of the Invention
The present invention relates to a marine structure comprising precoated corrosion resistant steel pipe piles for use in ports, harbors, the ocean, and rivers.
2. Description of the prior art
Steel pipe piles have been heretofore employed as foundation piles of a building on land and in harbors and river embankments and piers, and further, with regard to marine structures, have widely been used as steel building materials able to cope with deep water and poor ground.
In general, steel pipe piles for use in harbors, the sea, and rivers have been heretofore used without any covering. In recent years, however, harbor, sea and river structures are required to possess a durability over extended periods of 40 to 50 years.
It has thus become necessary to take anticorrosion measures enabling the steel pipe piles used in such harshly corrosive environments to maintain their corrosion-resistance for extended periods of time.
Various methods for preventing the corrosion of steel pipe piles have been known, such as use of a coating of tar-epoxy, electrical protection, and a mortar coating method employing fiber reinforced plastic (hereinafter referred to as FRP).
For instance, U.S. Pat. No. 3,417,569 discloses a protective coating for a steel pipe piling to be employed as an off-shore platform. The protective coating is said to protect the steel pipe piling from corrosion in the so-called splash zone, where corrosion is particularly severe. The coating is preferably a chloroprene polymer with a preferred thickness of 1/8 to 1 inch.
U.S. Pat. No. 2,874,548 discloses a steel piling for marine use surrounded by a layer of sealing material such as heavy dielectric grease or asphalt which, in turn, is surrounded by an imperforate cylindrical sleeve having high resistance to corrosion.
U.S. Pat. No. 2,791,096 discloses that the portion of a steel pile structure immersed in the splash and tidal zones is covered with a Monel (nickel-copper alloy containing 20-40% copper) sheet sheathing, and is cathodically protected from corrosion by the sea water below low tide by an anode electropositive to the steel structure.
U.S. Pat. No. 3,181,300 discloses the application of a thermoplastic flexible impervious sheet material in order to cover and embrace a steel pile therewith and further discloses bonding the joint of the thermoplastic sheet.
U.S. Pat. No. 3,321,924 discloses a method for protectively encasing a pile in situ which comprises forming a sheet of synthetic plastic material into a roll having a diameter less than the diameter of said pile, and expanding said roll about said pile, said roll contracting into close engagement with said pile, securing said roll to said pile, and sealing the space between said pile and said roll.
U.S. Pat. No. 3,370,998 relates to a method for preventing submerged pile structure from corrosion by sandblasting the corroded exterior thereof, preparing an epoxy resin sheet forming tray, and removing the resin sheet from the tray, and securing it to a plate so that the sheet and the plate may be applied to the pile.
U.S. Pat. No. 3,448,585 relates to a protective shield for pilings formed from a sleeve of heat-shrinkable plastic material encircling such pilings along the length thereof and heat-shrunk into tight encircling engagement about the piling to protect it from rot, rust, erosion, corrosion, insects or mechanical damage.
U.S. Pat. No. 3,505,758 relates to an antifouling protective cover for preventing the growth of marine organisms on the exterior of marine objects submerged in the sea water, and the cover consists of a double-wall, rubber-coated fabric reservoir which carries a diffusible toxic material.
U.S. Pat. No. 4,340,622 discloses that the blistering of an organic coating composition applied to a surface disposed adjacent to a body of water is avoided by applying the coating composition to the surface while water is flowing over the surface.
U.S. Pat. No. 4,283,161 relates to the method for protection of offshore flow line risers and existing platforms by wrapping such risers and platforms with a flexible membrane such as rubber, to protect such structures from damage due to waves and from biological growth.
U.S. Pat. No. 4,415,293 relates to the method for preventing marine growth on the shallow water portions of platform legs by applying a polymer coating to the legs and covering the polymer with a copper-nickel alloy anti-fouling covering. The legs are provided with sheaths made of copper-nickel alloy.
Canadian Pat. No. 465,358 relates to a pile which consists of a wood column surrounded by a concrete jacket. The wood column is provided with a plurality of annular notches, each of which is occupied by concrete rings formed integrally with the concrete jacket.
British Pat. No. 1,494,072 discloses that a structure partially immersed in water is protected against corrosion by placing a jacket around the structure and removing water from the space between jacket and structure. Polyurethane foam is injected into the space and sets to form a protective coating.
Netherlands Pat. No. 7,511,739 discloses a doubly coated steel pile having an internal layer of, for example, bitumen and an external layer of e.g. polyethylene. The tar-epoxy coating method is troublesome because it has to be repeated every few years. Furthermore, extended corrosion resistance cannot be expected, as even if the steel pipe pile is coated with the tar-epoxy before it is driven into place, the coating is soft and tends to be scratched when being handled or driven. In addition, after having been driven into place, it may be struck by driftwood or the like, causing damage to the coating, making the pile more susceptible to corrosion at that point. Moreover, if the steel pipe pile is coated with the tar-epoxy after it is driven into place, it follows that only the part above the water will be protected from corrosion. If the underwater portion of steel pipe pile is to be coated, the cost therefor would become very high because of the necessity of having to drain the water from around the pile. PG,7
On the other hand, however, the electrical corrosion protection measure is disadvantageous in that the electrochemical function is such that corrosion protection is difficult in the splash zone and the tidal zone, where steel corrosion develops most rapidly.
Corrosion of steel materials in harbors, seawater and rivers proceeds most rapidly in the splash zone and the tidal zone, and is slower underwater, and slower still on sea mud.
"Splash zone" in this specification refers to the portion above the mean high water mark obtained from the high point of the highest tide; "tidal zone" refers to the zone between the mean high water mark and the mean low water mark; and "seawater zone" refers to the portion below the mean low water mark.
According to a recent study on the corrosion rate of steel structures in harbors conducted by an official organization, the average corrosion rate of steel pipe pile is 0.37 mm/year to 0.6 mm/year in the splash zone, and 0.35 mm/year to 0.5 mm/year in the tidal zone and thereabout. It was found that the mean corrosion rate in the seawater zone tends to gradually decrease as the depth of the seawater increases, and it is less than 0.05 mm/year.
It was also reported that the corrosion rate was 0.1 mm/year to 0.05 mm/year in a riprap layer, 0.05 mm/year in sea sludge, and 0.01 mm/year to 0 mm/year in the sea mud.
Assuming a mean corrosion ratio of 1.0 in the splash zone and the tidal zone, the corrosion ratio in the seawater zone amounts only to about 1/10, and to only about 1/50 in the sea mud.
It follows from the above that electrical protection of corrosion is hardly effective for the steel pipe pile in the splash zone and the tidal zone where protection against corrosion is most desired. Consequently, as a very effective means for preventing corrosion in the splash and tidal zones, where corrosion is most marked, the following method has recently been proposed.
FIG. 1 of the accompanying drawings shows an embodiment of a conventional method of preventing corrosion.
In FIG. 1, in a steel pipe pile 1 driven into the sea bed 3, a FRP tubular cover 9 encloses the splash zone 4, the tidal zone 5, and part of the outer surface of the pile just below the tidal zone 5, with the space between the tubular cover 9 and the steel pipe pile 1 being filled with mortar 10. The lower end of the mortar 10 is covered by an anti-corrosion seal means 11. In FIG. 1, 7 is a concrete structure and 8 is a riprap layer.
FIG. 2 is an enlarged view of the principal portion of FIG. 1. To carry out the work of FIG. 1, a specialist, such as a diver, is required, and since the work is affected considerably by waves, tides, and other such marine conditions, the method is disadvantageous in that the working efficiency is so poor that there is insufficient waterproofness along the boundary between the concrete structure built onto the top of the steel pipe pile and the mortar filling. Moreover, the cost is high.